1. Field of the Invention
The present invention relates to systems and methods for multiplexing data from a plurality of data sources, and in particular to a method and system for substituting non-opportunistic auxiliary data for null data packets in a packetized data stream.
2. Description of the Related Art
Packet-based digital data transmission systems are now commonplace. One example of such systems are those which are used to transmit media programs such as television programs and audio data to subscribers via satellite. Such systems are required to transmit many (200 or more) channels of data to subscribers over a transmission system that has a fixed throughput.
By their nature, packet-based television systems must fill all packets with data, and the packets must fill the entirety of a constant bit rate (CBR) transmission data channel. Television and audio signals are “bursty” in that some pictures and some audio passages require more data to be faithfully reproduce the original signal than other pictures and passages. To account for this “bursty” nature and to assure the data from each of the channels are combined and transmitted within the throughput requirements of the transmission system, the data from each of the channels is typically provided to a statistical multiplexer. The statistical multiplexer supervises the encoders that encode and/or compress the incoming channels. This process includes negotiating and allocating throughput among each of the encoders. Statistical multiplexers are disclosed in U.S. Pat. No. 5,854,658, issued to Uz et al, U.S. Pat. No. 5,864,557, issued to Lyons, and U.S. Pat. No. 5,912,890, issued to Park, and U.S. Pat. No. 6,208,666, issued to Lawrence et al., which references are hereby incorporated by reference.
When the media program is reproduced by less data, null data packets are typically inserted into the data stream. This is performed so that the data channel is maintained at a CBR. While this technique is effective, the null packets used in this technique typically convey no useful information, and are therefore a waste of precious bandwidth. Hence, systems have been devised which insert opportunistic (non-time critical data) into the data stream before being output from the statistical multiplexer. The system disclosed in U.S. Pat. No. 5,864,557 is one such example.
However, since the number of null packets available at any given time is unpredictable, the only “opportunistic” data (data which is not time-critical) is transmitted in the place of the null data packets. Non-opportunistic data (with more stringent delivery timing requirements) cannot be transmitted using such methods. What is needed is a system and method for replacing null data packets with non-opportunistic auxiliary data. What is also needed is for the system and method that permits the insertion of null data packets without modification to the statistical multiplexer and to be operable with a wide variety of statistical multiplexer designs. The present invention satisfies that need.